By Chris Hogg
BBC News, Tokyo
If you are hooked on Sudoku - the puzzle where you fill numbers in on a grid - the man you have to blame is a Japanese publisher who unleashed it on an unsuspecting world.
Keep the rules simple, says puzzle master Maki Kaji
Maki Kaji collects new puzzles and refines them with the help of thousands of Japanese subscribers to his quarterly puzzles magazine.
Japan has many thousands of amateur puzzlers, but Kaji-san is in a different league.
He is the daddy of them all, a man whose business card says "Father of Sudoku" in case you were in any doubt of his importance.
It is a few years since he first published Sudoku, a game he had come across in the United States.
He gave it its name and helped to make it the worldwide success it is today.
"I get really moved when I see a new idea for a puzzle which has lots of potential," he said.
"I get really excited about it. It is like finding treasure. It's not about whether it will make money. It is purely the excitement of trying to solve it."
He said he did not make a lot of money out of Sudoku because he did not copyright the puzzle.
But he says that helped its success, because it meant others could pick up the idea and publish it more widely.
He now owns the copyright for the puzzle in Japan and still publishes new collections of the puzzles.
Enjoyment for everyone
But it is his unique relationship with those who read his puzzle books which has enabled him to create what you might describe as an "incubator" for new puzzles.
What do you think makes a good puzzle? Have you created 'the next Sudoku'?
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If there is ever going to be a "Son of Sudoku" it is likely to come from his company.
Four times a year he sells 50,000 copies of his puzzle magazine Nikoli.
Readers submit about 80% of the puzzles in the magazines.
They try to solve the puzzles, but they are also encouraged to improve or refine them.
New puzzles often take about a year, four editions, to get right, but they sometimes take more than two and a half years.
"The secret to inventing a good puzzle," he said, "is to make the rules simple and easy for everyone, including beginners.
"You have to be able to make both easy and difficult puzzles using the same rules.
"Between 200 and 300 people help to complete a new puzzle. It has to be something that children, old people and everyone in between can enjoy, to be really good."
Noboyuki Sakamoto, a young computer programmer, has had his puzzles published in the magazine.
"Japanese people are not good at inventing things," he said. "But we are very good at refining things.
"All the puzzles in this book share basic ideas as the base from which they are developed, but the readers make them more fun, they make them better.
"When I see new puzzles I can feel and understand what the puzzle's creator is thinking. At that moment I feel a sense of togetherness, I feel connected to them. It is pretty much like reading a novel I guess."
'Son of Sukoku'?
Prime time is puzzle time on Japanese TV. Shows like Boost Your IQ on Saturday night feature celebrities trying to solve brainteasers. The audience loves it.
So why do the Japanese enjoy number puzzles so much?
Tsuneharu Okabe, vice-president of the Japanese Mathematical Association, thinks it is because of the way they are taught at school.
But he also thinks that the complex Japanese language system means that crossword puzzles are much harder.
"We Japanese like mental arithmetic and we're good at it," he argued. "It's fairly easy for us to solve number puzzles like Sudoku.
"Its harder to do crossword puzzles. In Japanese there are many words which sound the same but have totally different meanings. The fact that Sudoku can have only one answer is very reassuring for us. We tend to get obsessed with it."
So what is going to be the next big thing - the "son of Sudoku"?
Maki Kaji showed me "Slitherlink", a new game in the latest issue of his magazine.
There is a grid made up of dots and the numbers 0, 1s, 2s and 3s scattered across it.
You have to join up the dots around the numbers but you can only use the same number of lines as the number they are next to.
You have to join them all up like a snake.
"This is fantastic," he beamed. "But it is really just for people who are puzzle fanatics. It's not like Sudoku which has universal appeal. Sudoku is enjoyed by people from five to 90."
But then his expression changed, becoming almost wistful.
"Sudoku is very, very special," he said. "The problem is something like Sudoku only comes along once every 100 years."
What do you think makes a good puzzle? Are you a puzzle fan? Have you created a puzzle you think could be the next Sudoku?
Please send your comments and game ideas to email@example.com